Unable to install add-ons to Blender 2.82a and Blender 2.9 on a Mac
The installation process is detailed under the GitHub page and is the same as most if not all add-ons.

Download the zip file from the releases page. Open Blender, open up Preferences, click on Add-On tab, press Install and select the zip file. You'll then be able to find the add-on listed as Mesh: EdgeFlow. I tested it. It works.